1. Field of the Invention
The present invention relates to an aluminum non-aqueous electrolyte secondary cell comprising an aluminum or an aluminum alloy negative electrode.
2. Description of the Related Art
In theory, aluminum yields an energy density per unit volume of 8,050 Ah/l. This value is about 4 times as large as that of lithium. Thus, a rechargeable cell of high energy density can be implemented at a low cost by using aluminum or an aluminum alloy for the negative electrode of the cell. Cells having an aluminum or an aluminum alloy negative electrode are therefore believed promising, and the development thereof is under way. An aluminum electrode yields a standard electrode potential of -1.66 V with respect to the standard hydrogen electrode. Thus, a secondary cell compatible with the conventional cells operating at an ordinary temperature, for instance, alkaline dry cells, silver oxide cells, nickel-cadmium batteries, nickel-hydrogen batteries, etc., are expected by using an aluminum negative electrode in combination with an appropriate positive electrode material. A secondary cell, particularly a secondary cell operating at room temperature, using an aluminum or an aluminum alloy negative electrode is therefore of great promise.
As compared with hydrogen, thermodynamics teaches that aluminum is by far stable against reduction. Accordingly, in case an aluminum electrode is employed, it is extremely difficult to implement an aluminum secondary cell using an electrolyte based on an aqueous solution. Furthermore, a stubborn and dense passive film is appeared on the surface of aluminum due to a strong affinity of aluminum with oxygen atoms. It can be readily understood that this leads to the formation of a large polarity, and to a further growth of a passive film on the negative electrode during discharge.
The development of a non-aqueous electrolyte is the key for developing a secondary cell which functions at room temperature using aluminum or an aluminum alloy as the negative electrode. It is also important to develop an activating material of the positive electrode suitable for the non-aqueous electrolyte.
The use of, for example, a non-aqueous electrolyte for lithium batteries based on an organic solvent, or such based on an ether or a high temperature molten salt has been proposed to the present for use as the non-aqueous electrolyte of secondary cell equipped with an aluminum or an aluminum alloy negative electrode. More recently, the use of a non-aqueous electrolyte based on a molten salt for ordinary temperature use comprising an aluminum halide/N-alkylpyridinium halide or an aluminum halide/N-alkylimidazolium halide is also proposed.
However, the non-aqueous electrolytes proposed to the present are disadvantageous in that they yield a conductivity lower than the conventional aqueous electrolytes by a digit or two. More specifically, for instance, in case the cell is built from a non-aqueous electrolyte based on an organic solvent for use in lithium cells and the like, the resulting cell suffers inferior drain capability due to the low conductivity of the electrolyte.
In case of a non-aqueous electrolyte based on ethers, moreover, not only the problems concerning conductivity, but also the inflammable nature of ether must be taken into account. More specifically, ether electrolytes must be handled with great care. In case of a non-aqueous electrolyte based on a high temperature molten salt, the temperature thereof must be maintained at 200.degree. C. or higher during the operation of the cell. In other words, the cell based on a high temperature molten salt does not function at room temperature. Considering a non-aqueous electrolyte based on a low temperature molten salt, there are many disadvantages concerning the stability of the molten salt. More specifically, molten salts not only are viscous, but also have a very limited temperature range of operation. If once the salt is brought at a temperature out of the limited range, the electrolyte readily undergoes solidification, or the construction or the concentration of the complex ions constituting the electrolyte change from the initial state.
Because no non-aqueous electrolyte particularly suitable for the aluminum non-aqueous electrolyte secondary cell is found to present, there is no concrete proposal for the material suitable for use as the positive electrode material.
In the light of the aforementioned circumstances, the present invention aims to overcome the problems of the prior art technology. Thus, an object of the present invention is to develop a non-aqueous electrolyte and a positive electrode material which enable reversible deposition and dissolution of aluminum at a high current density. It is also an object of the present invention to provide, by employing the non-aqueous electrolyte and the active material thus obtained, an aluminum non-aqueous electrolyte secondary cell with an aluminum or aluminum alloy negative electrode.